Download The Quest for Photocatalytic Systems with Broadband Solar

Photocatalysts
The Quest for Photocatalytic Systems with Broadband
Solar Absorption
Sanyang Han and Xiaogang Liu*
Since the first discovery of photocatalytic splitting of water
by Fujishima and Honda in 1972, the semiconductor catalysts
have played a central role in the conversion of solar energy
and the degradation of organic pollutants.[1] Over the past
few decades, the development of semiconductor photoelectrochemical systems for efficient solar energy conversion
has become the goal of numerous research efforts. Titanium
dioxide (TiO2) is perhaps the best investigated photocatalyst
in water detoxification and air pollution control by utilization
of solar energy. However, the photoactivity of TiO2 with a
bandgap energy of 3.2 eV is hampered by the requirement of
illumination with wavelength of less than 387 nm, which only
accounts for less than 5% of the solar energy. Another drawback associated with the TiO2 system is its relatively high
recombination rate of photoexcited charge carriers, thereby
resulting in a decrease in process efficiency.[2] To solve these
problems, a variety of strategies have been developed to
achieve efficient photocatalysts featuring absorption over
broad wavelength ranges.
One commonly used approach for enhancing the absorption efficiency of photocatalysts in the visible spectral region
involves bandgap engineering by doping metal and non-metal
species into TiO2 host lattices. For example, the bandgap
of the nitrogen-doped TiO2 can be narrowed by 0.72 eV as
a result of the hybridization of N(2p) and O(2p) energy
states.[3] The incorporation of other non-metal (such as C and
S) and metal elements (including Fe, Cu, Mn, Co, Pt, Ru, and
Au) is also used to activate photocatalytic reactions under visible light.[4] However, the improvement of the photocatalytic
activity by this approach is rather limited because the doping
process often enables enhanced charge carrier recombination.
To boost solar energy efficiency, dye-sensitized solar cells have
also been exploited to enhance the light absorption in the visible region.[5] Alternatively, semiconducting nanomaterials
S. Han, Prof. X. Liu
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543, Singapore
Fax: (+65) 6779 1691
E-mail: [email protected]
Prof. X. Liu
Institute of Material Research and Engineering
Agency for Science
Technology and Research
3 Research Link, Singapore, 117602, Singapore
DOI: 10.1002/smll.201302897
small 2014, 10, No. 7, 1243–1244
with narrow bandgaps, tunable optical absorption, and high
photostability can be utilized as sensitizers for visible light.
Importantly, the coupling of semiconductor sensitizers with
TiO2 in form of heteronanostructures could extend the energy
range of photoexcitation and significantly improve the photocatalytic efficiency by increasing the charge separation.[6]
Despite considerable efforts in developing heteronanostructured photocatalysts, the search for photocatalysts with broadband solar absorption covering the entire visible-NIR range
remains a formidable challenge.[7]
Writing in Advanced Materials, Liu and co-workers[8] now
describe an intriguing material design that allowed them to
trap the light within a remarkable wavelength range. The
material is based on a hybrid system comprising Bi2WO6
nanoplatelets and TiO2 nanobelts. Bi2WO6 is chosen as
the photocatalyst because of its narrow bandgap (2.8 eV)
that matches closely with the energy of visible light.[9] The
authors found that the Bi2WO6 nanoplatelets also possess
good photocatalytic properties under NIR irradiation. It
was believed that the presence of oxygen vacancies in the
Bi2WO6 nanoplatelets raises the Fermi level and reduces
the band edge, allowing for interband transitions and carrier
creation.[8] Transmission electron micrograph of the hybrid
semiconductor system shows the formation of a hybrid heteronanostructure with Bi2WO6 nanoplatelets grown epitaxially onto the surface of the TiO2 nanobelt (Figure 1a). The
Bi2WO6/TiO2 nanostructure exhibits a broadband enhancement of the light absorption and much improved photocatalytic performance for the degradation of methyl orange dyes
(Figure 1b,c).
The improvement in the photocatalytic performance of
the hybrid nanomaterials is partly attributed to the enlarged
surface area of the TiO2 nanobelt because of the conformal
growth of Bi2WO6 nanoplatelets. Apart from the increased
surface area, one must take into account the synergetic effect
imparted by combining the two components. A more likely
explanation is that the modification of the TiO2 nanobelt
with Bi2WO6 effectively suppresses the recombination rate of
photogenerated charge carriers.
To better understand the origin of the enhanced photocatalytic activity in Bi2WO6-modified TiO2 nanobelts, the energy
transfer process of the charge carriers needs to be considered.
As shown in Figure 2, both TiO2 and Bi2WO6 can be photoexcited by irradiation with UV light. Consequently, electrons
are excited across the band gap of the semiconductor where
no electron states exist. At the interface, the electron produced in the TiO2 nanobelt transfers to the conduction band
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1243
highlights
S. Han and X. Liu
valence band of Bi2WO6 can be easily transferred to that of
TiO2 because of their close energy levels, while the electron
produced in the Bi2WO6 conduction band from the excitation process remains in the Bi2WO6 particle, thereby enabling
the separation of charge carriers.
One of the major further challenges will be to improve
the photocatalytic activity of TiO2/Bi2WO6 nanostructure
under NIR excitation. An even greater challenge will be to
understand the interplay between surface and electronic
characteristics at the interface of the composite structure.
Achieving these ambitious goals may require the integration of spectral converting nanomaterials[10–13] to the existing
semiconductor systems. All in all, however, the work of Liu
and co-workers work should provide a new direction for the
development of broadband photocatalysts.
Figure 1. a) The HRTEM image of Bi2WO6/TiO2 heteronanostructures.
(Inset) The crystal structure of Bi2WO6 showing layers of corner-sharing
WO6 octahedral sheets and bismuth oxide sheets. b) The absorption
spectra of TiO2 nanobelts, Bi2WO6 nanoplatelets, and Bi2WO6/
TiO2 heteronanostructures. c) Photocatalytic performance for the
degradation of methyl orange in the presence of P25, TiO2 nanobelts,
Bi2WO6 nanosheets, and Bi2WO6/TiO2 heterostructures, respectively.
Reproduced with permission.[8] Copyright 2013, John Wiley &Sons.
of the Bi2WO6, while the photoinduced hole in the Bi2WO6
particle migrates to the TiO2 valence band. The transfer of
charge carriers between the TiO2 nanobelt and the Bi2WO6
particle increases the charge separation and thus efficiency
of the photocatalytic process (Figure 2a). In contrast, when
irradiated with visible-NIR light, only Bi2WO6 can be excited
to generate an electron to the conduction band owing to its
small bandgap (Figure 2b). The photogenerated hole in the
Figure 2. The proposed photoexcited charge transfer processes in the
Bi2WO6/TiO2 photocatalyst under a) UV and b) visible-NIR irradiation,
respectively.
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Acknowledgements
X.L. acknowledges the support from the National Research Foundation and the Economic Development Board (SPORE, COY-15-EWIRCFSA/N197-1), the Singapore-MIT Alliance, and the Agency for
Science, Technology and Research.
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 6, 2013
Published online: November 28, 2013
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